The present invention relates generally to photomasks useful in projecting an image. More particularly, the invention relates to erasable and reusable photomasks formed from photosensitive materials, including liquid crystal materials, and methods and apparatus for forming such photomasks.
It is known to use a photomask in projecting an image. Such a photomask can take a number of forms, including a photographically printed mask such as a photographic slide, or a xerographically printed mask such as an overhead projector transparency. Another known photomask uses a liquid crystal material. The liquid crystal material is normally transparent at ambient operating temperatures. Selective heating of small areas of the crystal creates light-scattering regions in the heated areas, thereby creating a pattern of scattering regions on an otherwise non-scattering background. Once the liquid crystal has been completely written, light may be projected through or reflected off the liquid cyrstal cell to project the image formed in the crystal.
One example of a liquid crystal photomask generating system can be seen in U.S. Pat. No. 4,564,853. A laser diode is used to apply writing heat to a liquid crystal cell to form a photomask through which visible light is later projected for printing an image on a photosensitive material. The laser beam emanating from the diode is directed onto X-Y scanning mirrors under the control of X-Y scanning galvanometers. Through proper control of the galvanometers, the beam is scanned over the entire area of the liquid crystal cell. By turning the laser beam on and off at appropriate times, data is written which forms the image in the liquid crystal photomask.
Other scanning arrangements for the laser beam are known. For example, in addition to a galvanometer scanner, a rotating drum scanner or acousto-optical scanner may also be used. Further, other photosensitive media than a liquid crystal are known which can be written upon by a laser beam to form an image.
Several disadvantages exist with any of these writing systems. The laser beam is focused onto the liquid crystal surface. However, the laser focal plane is not flat, and some form of focal plane correction is required to maintain focus at the writing surface. As a result, it is generally difficult to have very high resolution. Resolution is further degraded because the path length for the beam from the laser source to the liquid crystal must be relatively long to enable the beam to be scanned over the entire writing surface. Finally, known systems are generally appropriate only for use with a laser writing head having a single laser diode. Consequently, writing speed for such a system is generally slow.
What is needed, therefore, is a system for writing with a laser onto a photosensitive surface which overcomes the disadvantages of known systems as described above. Particularly, such a system should be of relatively simple construction, without requiring complicated devices for focal plane correction. Very high resolution in the written image should be possible. To increase writing speed, such a system should further be capable of use with multiple laser diodes.